Exiling cards face-down is poorly-supported in the engine; the closest it comes is to actually obliterate the cards in question, then re-create them when they return from exile, like Necropotence does. That doesn't work properly with the BFZ processor cards, though (which select cards from exile without any requirements on characteristics, so they should be able to pick face-down ones); and Warden of the Beyond is going to need a million and one hacks to even sort of see such cards. Commit 5beb7ad has the Shandalar implementation to get things right, but it relies on a very large amount of infrastructure that hasn't been ported to Manalink.